1. Field of the Invention
This invention relates to a signal composing circuit and, more particularly, to a signal composing circuit which is suited to a decoder device for a 4-channel matrix stereophonic system.
2. Description of the Prior Art
A decoder device for a 4-channel matrix stereophonic system requires a signal composing circuit which produces signals of predetermined coefficients or composed signals from a plurality of input signals in order to separate stereophonic signals from composite signals. The basic concept of the 4-channel matrix stereophonic system is disclosed by Ito et al in U.S. Pat. No. 3,825,684. An improvement thereof is described in Material No. EA72-32 (1973-03), "Improvements in Encode-Decode System in 4-channel Matrix Reproduction" which was published in the meeting of Denkionkyo Kenkyukai (Meeting of Technical Group of Electroacoustics) joint with Denshitsushin Gakkai (The Institute of Electronics and Communication Engineers of Japan), and with Nihon Onkyo Gakkai (The Acoustical Society of Japan) on Mar. 26, 1973 and which was issued by Denshitsushin Gakkai.
A circuit, which is composed of a plurality of resistors (called as coefficient resistors hereinafter) having one of the ends connected in common as an output terminal and the other ends connected to inputs signals, respectively, is generally known for composing the input signals. When such a circuit is used as a decoder device in a stereophonic system, the decoder device further requires phase inverters whose amplification factors are set at -1 and amplifiers (referred to as fixed coefficient amplifiers hereinafter) whose amplification factors are set at predetermined values such as -.sqroot.2, -2 and +2.
Such a known signal composing circuit, however, has problems as stated below.
1. In order to obtain inverted-phase signals, separate phase inverters which serve only for the function of the phase inversion are required.
2. In order to compose a plurality of signals, coefficient resistors are required, and the resistance value of the coefficient resistors must be made sufficiently lower than the input impedances of the fixed coefficient amplifiers connected to such coefficient resistors, whereas it must be made sufficiently higher than the output impedances of signal sources for driving the coefficient resistors.
3. Since separate coefficient resistors, phase inverters and fixed-coefficient amplifiers are necessary, the circuit arrangement of the signal composing circuit becomes very complicated for obtaining the sum component output signal and the difference component output signal from the plurality of input signals, and the number of the constituent circuits becomes large, with the result that the power consumption becomes high. Moreover, the resistance value of the coefficient resistors is comparatively large. Therefore, to construct the signal composing circuit of such a type into a monolithic semiconductor integrated circuit device (hereinafter termed the monolithic IC) becomes very difficult on account of the allowable power dissipation of the monolithic IC, the heat sink structure of the IC and the integration density of the IC.
4. Since in the respective signal paths from the inputs to the outputs of the signal composing circuit, there are disorderly arranged the coefficient resistors, the phase inverters, the fixed coefficient amplifiers and so forth, where such circuit components are D.C. coupled in order to form the signal composing circuit as an IC, deviations in the D.C. level temperature-dependency, the distortion factor characteristic and the frequency characteristic arise among the respective composite signal output voltages at the composed output terminals.
5. Since the respective signal paths have coefficient resistors with the comparatively high resistance connected in series, large thermal resistance noises are generated.